JJHerb@aol.com on fri 13 dec 96
Having been through the "S" crack discussion, the shivering discussion, the
crazing discussion, and the warping discussion, I would like to suggest that
most physical defects found in pottery pieces is caused by uncontrolled or
inappropriate volume changes. I have to exclude other glaze defects like
blisters and crawling but for warped, cracked, or broken pieces, think about
the volume changes that take place and the order in which they occur.
As an example, I had a few incidents of dunting (A crack across the foot of a
piece, not extending to the rim) when I had a greenware lady fire some thrown
pieces. She fired as she usually would but that broke some of my pieces
while it had never broken any of hers. My thought here is that all her slip
cast pieces were entirely uniform in thickness and, therefore, their
temperature upon heating and cooling would be uniform. My pieces had trimmed
feet that, while entirely appropriate for the pieces and elegant to boot,
were thicker than the rim. Her firing rates or cooling rates (more likely)
were fast enough that the uneven cooling - the interior of the thicker foot
stayed hot while the rest of the piece shrank - broke the pieces.
So, when you get crack or warps you don t like, think about the volume
changes going on in the various parts of the piece as it travels through your
work shop. If you get one you can t diagnose, blame me - I probably sneaked
over and hit it with a hammer.
Joseph Herbert
JJHerb@aol.com
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